A spin-transfer torque MRAM (Magnetic Random Access Memory) including a magnetoresistive element containing a ferromagnetic material as a memory element has been proposed. This MRAM is a memory that stores information by controlling the electrical resistance of the magnetoresistive element in two states, i.e., a high-resistance state/low-resistance state by changing the magnetization direction in a magnetic layer by a current to be injected into the magnetoresistive element.
The magnetoresistive element includes a storage layer as a ferromagnetic layer having a variable magnetization direction, a reference layer as a ferromagnetic layer having an invariable magnetization direction, and a tunnel barrier layer as a nonmagnetic layer formed between them.
A structure in which a perpendicular magnetization film having magnetic anisotropy perpendicular to the film planes is used as each of the storage layer and reference layer of the magnetoresistive element as described above has been proposed. When using the perpendicular magnetization film as the magnetoresistive element, no shape anisotropy is used, so the element shape can be made smaller than that when using an in-plane magnetization film. It is also possible to decrease the dispersion of the easy direction of magnetization in the perpendicular magnetization type magnetoresistive element. When adopting a material having a high magneto crystalline anisotropy, therefore, the perpendicular magnetization type magnetoresistive element is expected to achieve both micropatterning and a low current while maintaining a thermal disturbance resistance.
In the abovementioned perpendicular magnetization type magnetoresistive element, the storage layer and reference layer have different film structures. Also, the perpendicular magnetic anisotropy of the storage layer is lower than that of the reference layer. As the micropatterning of the magnetoresistive element advances, therefore, it is desirable to increase the perpendicular magnetic anisotropy of particularly the storage layer.